An electron beam addressed liquid crystal light valve of the cathode-ray tube type is described in Duane A. Haven, IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, 489-492, May 1983. The light valve of Haven is a form of cathode-ray tube (CRT) having a twisted nematic liquid crystal cell, one substrate surface of which serving as a target for a writing electron beam propagating in the tube. The target substrate comprises a thin sheet of dielectric material and forms one face of the liquid crystal cell.
The CRT also includes a writing electron gun, a flood electron gun, and a ring-type collector electrode positioned adjacent the periphery of the target surface. The flood electron gun maintains the target surface of the cell at a desired operating electrostatic potential V.sub.FG, which is the potential of the flood electron gun cathode. Polarized light propagating from an external source enters the CRT through an optically transparent entry window on one side of the tube and passes through the cell and out through an exit window. The writing and flood guns are mounted at oblique angles relative to the target substrate to keep them out of the light path. Unwritten areas of the liquid crystal cell remain in an "OFF" state that rotates by 90 degrees the polarization direction of the light emanating from the external source. Areas addressed by the writing beam are temporarily switched into an "ON" state that leaves unchanged the polarization direction of the light emanating from the external source and thereby creates a light image pattern that is detected by an analyzing polarizer positioned in the path of light exiting the exit window.
The collector electrode of the light valve of Haven is operated at a potential V.sub.col, which is positive relative to the potential V.sub.FG of the target surface. The flood gun electrons strike the target surface with an energy that is below the first crossover point on the secondary electron emission ratio curve for the dielectric material forming the target surface. Under these conditions, the electrostatic potential of the target surface is stabilized to the potential of the flood gun cathode. The writing gun is operated under conditions so that the writing beam electrons strike the target surface with an energy that is above the first crossover point but below the second crossover point of the dielectric material.
When the writing beam strikes the target surface, secondary emission causes the written area to charge positive relative to the unwritten areas of the target surface, which are at the flood gun potential V.sub.FG. The potential of the written area rises, approaching the potential V.sub.COL of the collector electrode and driving the liquid crystal cell into the "ON" state. After the writing beam is turned off, the potential drops back to the flood gun cathode potential V.sub.FG and allows the liquid crystal cell to relax to the "OFF" state. This occurs because V.sub.COL is below the first crossover point and more electrons are absorbed than are emitted from the previously written area.
The ring-type collector electrode is positioned adjacent the periphery of the liquid crystal cell and outside the projection light path through the valve. There is a relatively large separation between the collector electrode and the central areas of the target surface, which separation causes the collection of secondary electrons emitted from the central areas on the target surface to be relatively inefficient. The reason for such inefficiency is that secondary electrons emitted from the central areas on the target surface redeposit on the positively charged, previously written areas of the target surface. This redeposition of secondary electrons at least partly erases the written image, thereby reducing the resolution and contrast capability of the light valve.